The prevention and/or recovery of many types of fugitive dust occurring in industrial practices is of wide spread concern in today's society. Typical fugitive dusts are those of coal, cement, fly ash and minerals that are emitted to atmosphere by industrial activity including mining, transportation and manufacturing.
Fugitive dust is of much concern underground in mining operations such as in coal mines where it is known to lay such fugitive coal dust by spraying with aqueous systems containing chemical additives to improve working conditions and reduce the toxicological risks and explosion hazards. Most commonly used chemical additives are hydroscopic salt mixtures or surfactants (United Kingdom Pat. No. 677279 describes the use of aqueous systems containing alkyl benzene sulphonates; U.S. Pat. No. 3,900,611 utilizes the combination of an amphoteric surfactant with a sulfosuccinate surfactant in an aqueous solution of a polymer such as polyacrylamide; and, U.S. Pat. No. 4,425,252 describes the combination of sodium dodecylbenzene sulfonate and a polyethoxylated nonylphenol in water as a means for the rapid abatement of respiratory coal dust).
Above ground, the generation of fugitive dust arising out of coal fines in the surface handling of bulk coal, particularly in its transport, is of much public concern. When coal is being transported in open top gondola cars in unit trains under drying weather conditions, wind action due to rapid movement of the train tends to blow dust off the surface unless some means of retarding this action is provided. Settling of fugitive coal dust near the railway tracks, particularly in populated areas, is objectionable. As much as 45% of the coal that is handled in this way may be finer than 28 mesh. It has been proposed that the open top gondola cars be provided with covers to protect the coal against wind action. Use of fiberglass lids on cars carrying metal concentrates is known. Such lids add substantially to the tare weight of each shipment and their removal before dumping and filling of the cars requires much labor or elaborate mechanical equipment. For coal, the cost of covering is very high in relation to the value of the product shipped. An alternative is the laying of fugitive coal dust by coating of bulk coal with oils and binders as is illustrated by:
U.S. Pat. No. 2,431,891 which spray coats coal with an aqueous colloidal suspension of asphalt to render the coal substantially dustproof;
U.S. Pat. No. 4,169,170 which treats coal in open top hopper cars with asphalt emulsions or pulp mill black liquor compositions to reduce losses in transit due to the action of wind on the coal surface by realizing a coating depth penetration of from 6.4 to 15 cm, whereby a flexible crust of oily adhesiveness is realized; and,
U.S. Pat. No. 4,264,333 which covers the exposed surface of bulk or piled-up coal with a flexible, water resistant and force resistant blanket of coal tar by a two step operation in which the exposed surface of the coal is prewetted with a wetting agent and immediately thereafter the coal tar emulsion is sprayed on the coal.
The principal means relied on for abatement of particulate emissions, including fugitive dust, from manufacturing facilities are cyclones, electrostatic precipitators, fabric filters, and scrubbers. The widely used fabric filter systems, i.e., bag houses, usually consist of tubular bags made of woven synthetic fabric or fiberglass, in which the dirty gases pass through the fabric while the particles are collected on the upstream side by the filtering action of the fabric whereby the fugitive dust is removed from the ambient environment. The fugitive dust retained on the bags is periodically shaken off and falls into a collecting hopper for recovery. Bag houses provide high collection efficiency (an essential under current environmental laws) with wide application in many industries, including mining operations, food processing, grain elevators, soap and detergents, plastics manufacture, manufacture of carbon black and cement, and used in combination with the operation of electric arc furnace, foundry cupolas and nonferrous smelters.
Unfortunately, in addition to the disadvantages of high initial cost and high maintenance costs for bag replacement, removal and disposal of the retained dust without regeneration of fugitive dust is difficult to impossible and remains a critical problem requiring a solution.
Illustrative of a particularly difficult fugitive dust problem is that which arises out of the handling of calcined coke. Coke is a carbonaceous material obtained from the carbonization of coal or refinery crude oil bottoms into particulate solids useful for iron ore sintering, boiler firing, electric smelting, chemicals manufacturing and other purposes. Most coke producers market several coke sizes which involve crushing followed by screening separation into the desired sizes. The resulting sized green coke is subjected to calcination when it is desired to produce a densified coke of high purity as is required for its processing into a carbon electrode useful for metallurgical applications. The calcined coke is maintained in a closed environment during its crushing, screening, storage and transport. Since the particulate calcined coke is fragile, its movement generates calcined coke dust that is collected through an air recovery/bag house system. When the bag house dumps the collected calcined coke dust, it is discharged either to a waste container or back onto the coke being transported which compounds the dust load. The disposal is difficult, cumbersome and wasteful of an otherwise useful product whereas return of collected dust increases the likelihood of its objectionable release to the outside atmosphere.
Patents have described the use of aqueous or asphalt emulsions and surface active agents such as ethoxylated alkylphenols for dust control of various minerals including coal or fly ash and sand including: Japanese Pat. No. 53085819 utilizes an asphalt emulsion containing spent sulfide solution obtained from the pulp industry and containing a polyoxyethylene-nonylphenol ether as a dustproof material binder; Japanese Pat. No. 50040491 prevents dust erosion of coal or ore piles by the application of an asphalt emulsion and a mixture of an anionic and nonionic surfactant (polyethylene glycol nonylphenol ether) by spraying this system onto powdered coal; German Pat. No. 2002364 binds dust in coal mines with a wetting agent such as alkylphenol polyglycol ether in water; and Japanese Pat. No. 56067385 dust proofs coal, ore, fly ash, etc. by applying to particular product a mixture of a nonionic surfactant (such as polyoxyethylene alkyl ether) and a polyhydric alcohol.
Unfortunately, none of these dust control references offer a suitable solution for the recovery of fugitive dust, particularly coke dust, for today's environments. Perhaps this inadequacy is because the coke is neither mineral nor coal, both by definition and wetting characteristics. Calcined coke appears even more difficult to water wet than coke. It is believed that, unfortunately, none of the above procedures satisfactorily eliminate the fugitive (airborne) dust that tends to be recirculated within the transport shed, the dust as recirculated on the belt during transport operations, the dust that must be collected and subsequently removed to a landfill, and/or the fugitive dust which escapes into the atmosphere adjacent to the loading system as a pollutant.
It is therefore an object of the invention to provide a process for recovery of fugitive dust, particularly that derived from calcined coke, and provide a method for collection of the fugitive dust into a useful product.
Other objects will be apparent from the disclosure of this invention which follows hereafter.